1. Field of the Invention
The present invention relates to a mode locked lasers and more particularly to a mode locked laser that can be used to produce relatively high-power output pulses which includes a plurality of optical amplifiers coupled to a common resonant cavity.
2. Description of the Prior Art
Mode locked lasers (MLL) are generally known in the art. Examples of such MLLs are disclosed in U.S. Pat. Nos. 3,978,429; 4,019,156; 4,435,809; 4,665,524; 5,764,679; 5,802,084; and 5,812,308. Such MLLs are used in various optical signal processing applications as well as in relatively high-speed optical communication systems. Such MLLs generate a periodic series of output pulses whose repetition rate is related to the round-trip pulse transit time within the laser cavity.
Various types of laser mode locking are known; such as active, passive and hybrid mode locking. An example of an active mode locked laser is disclosed in U.S. Pat. No. 4,019,156, while passive mode locked lasers are disclosed in U.S. Pat. Nos. 3,978,429 and 4,435,809. Active mode locking accomplished by way of an external modulation circuit which has a repetition rate or frequency which matches the laser round-trip cavity length. For example, an electrical pulse train can be applied to a laser diode, wherein the frequency of the pulse train is related to the round-trip cavity length. An example, of a passive laser is disclosed in U.S. Pat. No. 3,978,429. Such passive mode locked lasers are known to include a saturable reactor within the resonant cavity. Hybrid mode locked lasers are also known. An example of hybrid mode locked laser is disclosed in U.S. Pat. No. 4,019,156.
Semiconductor diode lasers are known to be used in such MLLs for optical signal processing as well as optical communication systems. Unfortunately, the output power of such semiconductor diode lasers is rather limited. In order to increase the output power of such mode locked lasers, post amplification is used. More particularly, an optical amplifier is disposed in the laser cavity and operated in such a manner that a large number of longitudinal cavity modes are mutually coupled together to achieve phase coherence. For example, U.S. Pat. No. 5,802,084 discloses a mode locked semiconductor laser combined with a tapered semiconductor optical amplifier, whose input is optically coupled to the output of the mode locked semiconductor laser. The tapered semiconductor optical amplifier increases the power output in order to provide relatively short high-peak power pulses. Unfortunately, the maximum pulse energy is limited to the peak power handling capability of the optical amplifier. Thus, there is a need for a semiconductor mode locked laser with increased power output pulses that does not depend on the power handling capability of an optical amplifier.
The present invention relates to a mode locked semiconductor laser that can generate relatively high-power output pulses. The semiconductor mode locked laser is formed from an array of optical amplifiers that are coupled to a common resonant cavity by way of an optical channelizer or wavelength division multiplexer (WDM). By utilizing an array of optical amplifiers, the output pulses will have a relatively higher power than known mode locked lasers. As such, the mode locked semiconductor laser in accordance with the present invention does not depend on the power handling capability of an individual optical amplifier and allows the average output power to be scaled, for example, up to the kilowatt range using a plurality of semiconductor optical amplifiers. A mode locked laser in accordance with the present invention can be implemented with active mode locking, passive mode locking or a hybrid of the two. The output pulse shape can be tailored by the amplifier drive current distribution.